In microwave communication networks, there is always a challenge to obtain good performance and capacity for a given communications protocol, its parameters and the physical environment in which the microwave communication network is deployed.
Some microwave link based networks implement the possibility to utilize so-called adaptive modulation. According to adaptive modulation the transmission rate of a microwave link is adapted to current propagation conditions in real-time; when channel conditions of the link are favorable the transmission rate is increased and when conditions of the link are not favorable transmission (at high rate) is decreased. In terms of protocol stack levels, adaptive modulation is associated with the physical layer (also denoted layer one).
When using adaptive modulation the maximum transmission rate is not limited by worst case conditions as is the case in systems with a static (i.e. non-adaptive) transmission rate and therefore the average link throughput in terms of bits per second and hertz can be significantly increased in many cases. The changes in rate are hitless, i.e., they occur without any bit errors in the forwarded traffic. Since this is a layer one technology, transmission rate changes can be performed rapidly, hence resulting in significant changes in bandwidth over short time durations (in the order of tens of milliseconds for a microwave backhaul application). As herein used, the term bandwidth refers to various bit-rate measures, representing the available or consumed data communication resources expressed in bits/second.
Microwave based mobile backhaul networks may be provided with path redundant topologies. Necklace, ring and meshed topologies are examples being deployed all the way to the cell sites. This means that several possible paths exist when traversing a network from point A to point B.
Combining redundant topologies with un-coordinated adaptive modulation on individual links yields a network where the optimal paths through the network are dependant on the current radio conditions on the links and therefore may change rapidly over time. Small temporal bandwidth changes can to some extent be smoothed out by buffering. In case of longer periods of altered bandwidth, traffic should be switched to an alternate path with available bandwidth that can sustain the traffic.
Within ITU-T SG15 there is an effort triggered to further elaborate on protection mechanism taking into account adaptive bandwidth links. Several proposed ways forward exist, all with the commonality that the granularity of the mechanisms above should be increased. However, there is a need for improved microwave link control in microwave communication networks.